1. Field of the Invention
The present invention relates to an image display apparatus called a head-mounted display (HMD) or the like or used as the viewfinder of a camera.
2. Related Background Art
It is required that the overall size of such an image display apparatus be reduced. In order to meet this requirement, various optical systems have been proposed.
For example, the present applicant has proposed image observation apparatuses each designed to reduce the overall apparatus size by using a liquid crystal display for displaying image information and a compact prism-like optical element for projecting the image, displayed on the liquid crystal display, onto the eyes of an observer to allow him/her to observe it in Japanese Patent Application Laid-Open Nos. 7-333551 (corresponding to EP 0687932A3), 8-50256, 8-160340, and 8-179238.
According to the image display apparatus proposed in each reference described above, light from the liquid crystal display that is displaying image information is guided to the eyes of an observer via a refracting surface, total-reflecting surface, and reflecting surface having curvatures of a compact prism-like optical element, thereby forming a virtual image on the liquid crystal display in front of the observer and allowing the observer to observe the virtual image.
In those times when these image display apparatuses were proposed, transmission type liquid crystal displays were generally used as liquid crystal displays. However, a transmission type liquid crystal display has a low pixel aperture ratio, and hence it is difficult to achieve an increase in the resolution of an image display element and a reduction in size, which have recently been required, while maintaining high image quality.
Under the circumstances, techniques of using a reflection type liquid crystal display having a high pixel aperture ratio for an image display apparatus required to display high-resolution images have been proposed. For example, an image display apparatus using a reflection type liquid crystal display is disclosed in Japanese Patent Application Laid-Open No. 11-125791. In this apparatus, as shown in FIG. 11 in this specification, a reflection type liquid crystal display 108 is directly illuminated with light from a light source 112 without the mediacy of any optical element.
In this image display apparatus, however, since light from the light source 112 is directly incident on the reflection type liquid crystal display 108, an illumination unit including the light source 112 and reflection type liquid crystal display 108 and the overall apparatus increase in size. In addition, since the reflection type liquid crystal display 108 greatly tilts with respect to a display optical system 110, the optical path lengths from the reflection type liquid crystal display 108 to the display optical system 110 greatly vary depending on places, resulting in a deterioration in optical performance.
According to another image display apparatus disclosed in Japanese Patent Application Laid-Open No. 11-125791, as shown in FIG. 12, a light source 112 is placed on the opposite side of a liquid crystal display 108 with respect to a prism-like display optical system 110. In this apparatus, light form the light source 112 is transmitted through the prism-like display optical system 110 and illuminates the liquid crystal display 108. Of the illumination light, the light (image light) reflected by the liquid crystal display 108 enters the display optical system 110 again to reach an eye 101 of an observer.
In this illumination system, however, unnecessary reflection in the prism-like display optical system 110 will produce flare light that is a big factor for a deterioration in image quality.
It is an object of the present invention to provide an image display apparatus using a reflection type display element suited to high-resolution images such as PC outputs, in which various aberrations can be properly corrected with a very simple, compact arrangement.
In order to achieve the above object, an image display apparatus according to an embodiment of the present invention is characterized by comprising a light source for supplying illumination light, a reflection type display element for modulating the illumination light into image light by reflecting the light, an illumination optical system for guiding the illumination light to the reflection type display element, and a projection optical system for guiding the image light to an observer, wherein the apparatus includes a member (correcting member) for correcting an optical path length difference between light beams of the image light which is caused when the image light is incident on the projection optical system via at least a portion of the illumination optical system.
More specifically, at least one of conditions given by
t1 less than t2
"PHgr"1 greater than "PHgr"2
is preferably satisfied when a ray connecting an exit pupil center of the projection optical system and a center of the reflection type display element is defined as a reference axis ray, in a plane including the reference axis ray, a ray of the image light having a maximum optical path length when the image light passes through at least a portion of the illumination optical system is represented by r1 and a ray of the image light having a minimum optical path length when the image light passes through at least a portion of the illumination optical system is represented by r2, optical path lengths of the rays r1 and r2 in the correcting member are respectively represented by t1 and t2, and optical powers exerted on the rays r1 and r2 by the correcting member are respectively represented by "PHgr"1 and "PHgr"2.
In this case, assuming that the correcting member has two optical surfaces A and B, if optical powers of the optical surfaces A and B are respectively represented by xcfx86i(A) and xcfx86i(B), a refractive index of a material for the correcting member is represented by nd, the optical surface A is located on the observer side, the optical surface B is located to oppose the optical surface A, a sign of a radius of curvature of that optical surface (A, B) is positive when the center of radius of curvature of that optical surface exists in the reflection type display element side, and local radii of curvatures at intersections of the light beam ri and the optical surfaces A and B are respectively represented by Ri(A) and Ri(B) given by
xcfx86i(A)=(ndxe2x88x921)/Ri(A) 
xcfx86i(B)=(1xe2x88x92nd)/Ri(B) 
then an optical power xcfx86i of the light beam ri is defined by
"PHgr"i=xcfx86i(A)+xcfx86i(B)xe2x88x92"PHgr"i(A)xc3x97xcfx86i(B)xc3x97ti/nd